Drink plenty of fluids throughout the day. Preparing to sing requires a holistic approach that does not confine itself simply to the moments before you go on stage. Take care of your vocal cords throughout the day to keep them in top condition.
Avoid drinking caffeine before you sing or perform. Caffeine dries out the vocal cords, making them less flexible and elastic.
Sing an exercise that uses the sound "hmmmm" by starting on a note in the middle of your range and singing down a perfect fifth, or five steps on the piano. Add a minor second each time, and continue singing this exercise until the highest point of your range. This will improve your flexibility and prepare your vocals for singing extended works.
Buzz by allowing the lips to vibrate freely on the notes of a C major scale. Continue up and down the scale and then start again a half step higher. Perform this exercise on several scales to help warm up your voice and prepare to sing.
Practice singing chorales and songs that stay in the middle of your range as a warm-up. The key is to prevent yourself from singing anything extended and to warm up and stretch the vocal cords until thoroughly warmed up for singing preparation.